Lecture 7
Lecture 7
I Absorbance (A) = log
o
I %Transmission (T) =
I X
100
Io
Beer-Lambert Law: A = ε c l
Infrared Spectroscopy
Summary
IR measures vibrational transitions Can be described by classical oscillator theory – Frequency proportional to [ bond strength/mass]1/2 – Except that only discrete states are allowed
Characteristic Group Frequencies are key – OH and C=O are particularly easy to identify – What is not there is as important as what is there
Know how to read the chart…cm-1?? Practice at the online sites…nmr, IR..Excellent!! You will have “unknowns” on the midterm exam
UV-Vis
Monochromator
Detector
Lens Lamp
Sample
UV/Visible Spectroscopy
Most UV/visible spectrophotometers cover from – 200 to 400 nm (the near ultraviolet) and – 400 nm (violet light) to 700 nm (red light) Region of Spectrum
Wavelength (nm)
Energy (kcal/mol)
ultraviolet
200-400
71.5 - 143
visible
400-700
40.9 - 71.5
C-C bond Disassociation energy ~ 95Kcal/mol
UV/Vis Spectroscopy
λmax
Wavelength (nm)
Transmission
UV-Vis spectral data are plotted as absorbance (A) versus wavelength (nm) Absorbance (A)
UV/Vis Spectroscopy
Absorbance: a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation at a particular wavelength Absorbance (A) = log
Io I
Where: I0 is the intensity of the incident radiation on the sample I is the intensity transmitted through the sample
UV/Vis Spectroscopy
Transmission: a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation at a particular wavelength I %Transmission (T) = Io
X
100
Where: I0 is the intensity of the incident radiation on the sample I is the intensity transmitted through the sample
Beer-Lambert law
Beer-Lambert law: the relationship between absorbance, concentration, and length of the sample tube
Beer-Lambert Law: A = ε c l A = absorbance c = concentration (mol • liter -1 ) l = length of the sample tube (cm) ε = molar absorptivity (liter • mol-1 • cm -1 ). Experimental values of ε range from 0 to 106
Origin of UV-Vis Absorbance MO Theory
∆E = 64Kcal/mole
Origin of UV-Vis Absorbance MO Theory
H
C C H H H Ethylene
Electronic Transitions
Absorption of UV-vis radiation results in transition of electrons from a lower energy occupied MO to a higher energy unoccupied MO For example, π to π * transitions in conjugated systems such as O
CH 2=CH-CH=CH 2 1,3-Butadiene
CH2 =CH-C-CH 3 3-Buten-2-one
O CH Benzaldehyde
Electronic Transitions
Transitions between vibrational and rotational energy levels are superimposed on the electronic excitations
The result is a large number of UV-Vis absorption peaks so closely spaced that the spectrophotometer cannot resolve them
For this reason, UV-Vis absorption peaks usually are much broader than IR peaks
Electronic Transitions
Wavelengths and energies required for π to π * transitions of ethylene and three conjugated (alternating) polyenes The “longer the wire, the redder the absorbance”
Name ethylene 1,3-butadiene
Structural Formula CH2 =CH 2 CH2 =CHCH=CH 2
CH2 =CHCH=CHCH=CH 2 (3E)-1,3,5hexatriene (3E, 5E)-1,3,5,7- CH2 =CH(CH=CH) 2 CH=CH 2 octatetraene
Energy λmax (kcal/ (nm) mol) 165 173 217 132 268 107 290
92
Summary
Know how to estimate the energetics of the radiation and the transitions per example 23.9 on page 910 Know how to use UV-Vis to do quantitative analysis (example 23.10 on page 912) We will work a lot more on molecular orbitals in the next section…but review pages 31-42
Chemistry 618B
Some History 1825 Michael Faraday isolates a new hydrocarbon from illuminating gas. 1834 Eilhardt Mitscherlich isolates same substance and determines its empirical formula to be CnHn. Compound comes to be called benzene. 1845 August W. von Hofmann isolates benzene from coal tar. 1866 August Kekulé proposes structure of benzene.
Greatest Scientist in History??
Michael Faraday (1791-1867)
What in the World is Benzene??
C6H6 discovered by Michael Faraday in 1823 – Synthesized in 1834 from benzoic acid – Remarkable chemical stability – Unsaturation number is very high but….
Does not add Bromine Substitutiuon with Br2 / FeBr3 Not oxidized by Permanganate or ozone No reaction with strong HBr (aq) No reaction with Hydrogen on Pd..??????
Addition vs Substitution…a Mystery Br +
Br2 Br
Addition of Bromine to Cyclohexene Br FeBr3 +
+
Br2
Substitution of Bromine for Hydrogen in Benzene
C6H6 + Br2
C6H5Br + HBr
HBr
∆H° of Hydrogenation Name
Structural Formula
∆H° (kcal/mol)
ethylene propene 1-butene cis-2-butene trans-2-butene 2-methyl-2-butene
CH2 =CH 2 CH3 CH=CH 2 CH3 CH2 CH=CH 2 CH3 CH=CHCH 3 CH3 CH=CHCH 3 (CH 3 ) 2 C=CHCH 3
-32.8 -30.1 -30.3 -28.6 -27.6 -26.9
2,3-dimethyl-2-butene
(CH 3 ) 2 C=C(CH 3 ) 2
-26.6
A quantitative measure of the special stability of benzene
Friedrich August Kekulé (1829-1896)
QUESTION: Can you tell me more about this vision to which you have referred? KEKULÉ: Surely! Let me read to you the remarks I am about to make to the assembly today: During my stay in London I resided in Clapham Road....I frequently, however, spent my evenings with my friend Hugo Mueller....We talked of many things but most often of our beloved chemistry. One fine summer evening I was returning by the last bus, riding outside as usual, through the deserted streets of the city....I fell into a reverie, and lo, the atoms were gamboling before my eyes. Whenever, hitherto, these diminutive beings had appeared to me, they had always been in motion. Now, however, I saw how, frequently, two smaller atoms united to form a pair: how a larger one embraced the two smaller ones; how still larger ones kept hold of three or even four of the smaller: whilst the whole kept whirling in a giddy dance. I saw how the larger ones formed a chain, dragging the smaller ones after them but only at the ends of the chains....The cry of the conductor: "Clapham Road," awakened me from my dreaming; but I spent a part of the night in putting on paper at least sketches of these dream forms.
Friedrich August von Kekule had a dream of whirling snakes, and the structure of benzene. He reported the dream in the following words in 1890, in a speech at a dinner commemorating his discovery that reportedly occurred during a “reverie” on a London bus.
Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen. Arthur Koestler (in "The Act of Creation") called this incident "probably the most important dream in history since Joseph's seven fat and seven lean cows.
H H
H
H
H H
Some Alternative Structures for Benzene H H2C
C
C
C H
C
CH2
Contributors to our understanding of the Structure of Benzene
1791-1867
1829-1896
1901-1994
Benzene - Kekulé
August Kekulé proposed a structure for benzene in 1872 H
H H
H
H
H H
H H
H
H H
This structure, however, did not account for the unusual chemical reactivity of benzene
•Kekule’s proposal is an equilibrium between two structures Br
Br Br
Br
•Resonance Theory (Pauling) describes resonance structures generated by electron movement (only!) that are not real, they are constructs the weighted sum of which describes the real molecule, which is presented as the resonance hybrid
Br
Br
Br
Br Br
“Resonance hybrid” Br
Benzene - Resonance
We can represent benzene as the hybrid of two equivalent Kekulé structures – each makes an equal contribution to the hybrid, and thus the C-C bonds are neither double nor single, but something in between
You need to understand how to “Push Arrows” If this does not make sense to you….PLEASE …get help immediately and don’t feel “dumb” O
O
Benzene - Resonance
Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds
One way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene that we saw earlier
This theory provides an explanation for the reactivity of benzene
Kekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6 with alternating single and double bonds. H H
H
H
H H
Kekulé Formulation of Benzene Note bond lengths
H
H H H H
H
H
H
H
H
H H
Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. H H H
H
H
H
H
H
H
H
H
H
Kekulé Formulation of Benzene However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. X X H
X
H
X
H
H
H
H
H
H
Structure of Benzene Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length.
Benzene has the shape of a regular hexagon.
All C—C bond distances = 140 pm
140 pm 140 pm 140 pm
140 pm
146 pm
140 pm 140 pm
134 pm
140 pm is the average between the C—C single bond distance and the double bond distance in 1,3butadiene.
I Absorbance (A) = log
o
I %Transmission (T) =
I X
100
Io
Beer-Lambert Law: A = ε c l
Infrared Spectroscopy
Summary
IR measures vibrational transitions Can be described by classical oscillator theory – Frequency proportional to [ bond strength/mass]1/2 – Except that only discrete states are allowed
Characteristic Group Frequencies are key – OH and C=O are particularly easy to identify – What is not there is as important as what is there
Know how to read the chart…cm-1?? Practice at the online sites…nmr, IR..Excellent!! You will have “unknowns” on the midterm exam
UV-Vis
Monochromator
Detector
Lens Lamp
Sample
UV/Visible Spectroscopy
Most UV/visible spectrophotometers cover from – 200 to 400 nm (the near ultraviolet) and – 400 nm (violet light) to 700 nm (red light) Region of Spectrum
Wavelength (nm)
Energy (kcal/mol)
ultraviolet
200-400
71.5 - 143
visible
400-700
40.9 - 71.5
C-C bond Disassociation energy ~ 95Kcal/mol
UV/Vis Spectroscopy
λmax
Wavelength (nm)
Transmission
UV-Vis spectral data are plotted as absorbance (A) versus wavelength (nm) Absorbance (A)
UV/Vis Spectroscopy
Absorbance: a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation at a particular wavelength Absorbance (A) = log
Io I
Where: I0 is the intensity of the incident radiation on the sample I is the intensity transmitted through the sample
UV/Vis Spectroscopy
Transmission: a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation at a particular wavelength I %Transmission (T) = Io
X
100
Where: I0 is the intensity of the incident radiation on the sample I is the intensity transmitted through the sample
Beer-Lambert law
Beer-Lambert law: the relationship between absorbance, concentration, and length of the sample tube
Beer-Lambert Law: A = ε c l A = absorbance c = concentration (mol • liter -1 ) l = length of the sample tube (cm) ε = molar absorptivity (liter • mol-1 • cm -1 ). Experimental values of ε range from 0 to 106
Origin of UV-Vis Absorbance MO Theory
∆E = 64Kcal/mole
Origin of UV-Vis Absorbance MO Theory
H
C C H H H Ethylene
Electronic Transitions
Absorption of UV-vis radiation results in transition of electrons from a lower energy occupied MO to a higher energy unoccupied MO For example, π to π * transitions in conjugated systems such as O
CH 2=CH-CH=CH 2 1,3-Butadiene
CH2 =CH-C-CH 3 3-Buten-2-one
O CH Benzaldehyde
Electronic Transitions
Transitions between vibrational and rotational energy levels are superimposed on the electronic excitations
The result is a large number of UV-Vis absorption peaks so closely spaced that the spectrophotometer cannot resolve them
For this reason, UV-Vis absorption peaks usually are much broader than IR peaks
Electronic Transitions
Wavelengths and energies required for π to π * transitions of ethylene and three conjugated (alternating) polyenes The “longer the wire, the redder the absorbance”
Name ethylene 1,3-butadiene
Structural Formula CH2 =CH 2 CH2 =CHCH=CH 2
CH2 =CHCH=CHCH=CH 2 (3E)-1,3,5hexatriene (3E, 5E)-1,3,5,7- CH2 =CH(CH=CH) 2 CH=CH 2 octatetraene
Energy λmax (kcal/ (nm) mol) 165 173 217 132 268 107 290
92
Summary
Know how to estimate the energetics of the radiation and the transitions per example 23.9 on page 910 Know how to use UV-Vis to do quantitative analysis (example 23.10 on page 912) We will work a lot more on molecular orbitals in the next section…but review pages 31-42
Chemistry 618B
Some History 1825 Michael Faraday isolates a new hydrocarbon from illuminating gas. 1834 Eilhardt Mitscherlich isolates same substance and determines its empirical formula to be CnHn. Compound comes to be called benzene. 1845 August W. von Hofmann isolates benzene from coal tar. 1866 August Kekulé proposes structure of benzene.
Greatest Scientist in History??
Michael Faraday (1791-1867)
What in the World is Benzene??
C6H6 discovered by Michael Faraday in 1823 – Synthesized in 1834 from benzoic acid – Remarkable chemical stability – Unsaturation number is very high but….
Does not add Bromine Substitutiuon with Br2 / FeBr3 Not oxidized by Permanganate or ozone No reaction with strong HBr (aq) No reaction with Hydrogen on Pd..??????
Addition vs Substitution…a Mystery Br +
Br2 Br
Addition of Bromine to Cyclohexene Br FeBr3 +
+
Br2
Substitution of Bromine for Hydrogen in Benzene
C6H6 + Br2
C6H5Br + HBr
HBr
∆H° of Hydrogenation Name
Structural Formula
∆H° (kcal/mol)
ethylene propene 1-butene cis-2-butene trans-2-butene 2-methyl-2-butene
CH2 =CH 2 CH3 CH=CH 2 CH3 CH2 CH=CH 2 CH3 CH=CHCH 3 CH3 CH=CHCH 3 (CH 3 ) 2 C=CHCH 3
-32.8 -30.1 -30.3 -28.6 -27.6 -26.9
2,3-dimethyl-2-butene
(CH 3 ) 2 C=C(CH 3 ) 2
-26.6
A quantitative measure of the special stability of benzene
Friedrich August Kekulé (1829-1896)
QUESTION: Can you tell me more about this vision to which you have referred? KEKULÉ: Surely! Let me read to you the remarks I am about to make to the assembly today: During my stay in London I resided in Clapham Road....I frequently, however, spent my evenings with my friend Hugo Mueller....We talked of many things but most often of our beloved chemistry. One fine summer evening I was returning by the last bus, riding outside as usual, through the deserted streets of the city....I fell into a reverie, and lo, the atoms were gamboling before my eyes. Whenever, hitherto, these diminutive beings had appeared to me, they had always been in motion. Now, however, I saw how, frequently, two smaller atoms united to form a pair: how a larger one embraced the two smaller ones; how still larger ones kept hold of three or even four of the smaller: whilst the whole kept whirling in a giddy dance. I saw how the larger ones formed a chain, dragging the smaller ones after them but only at the ends of the chains....The cry of the conductor: "Clapham Road," awakened me from my dreaming; but I spent a part of the night in putting on paper at least sketches of these dream forms.
Friedrich August von Kekule had a dream of whirling snakes, and the structure of benzene. He reported the dream in the following words in 1890, in a speech at a dinner commemorating his discovery that reportedly occurred during a “reverie” on a London bus.
Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen. Arthur Koestler (in "The Act of Creation") called this incident "probably the most important dream in history since Joseph's seven fat and seven lean cows.
H H
H
H
H H
Some Alternative Structures for Benzene H H2C
C
C
C H
C
CH2
Contributors to our understanding of the Structure of Benzene
1791-1867
1829-1896
1901-1994
Benzene - Kekulé
August Kekulé proposed a structure for benzene in 1872 H
H H
H
H
H H
H H
H
H H
This structure, however, did not account for the unusual chemical reactivity of benzene
•Kekule’s proposal is an equilibrium between two structures Br
Br Br
Br
•Resonance Theory (Pauling) describes resonance structures generated by electron movement (only!) that are not real, they are constructs the weighted sum of which describes the real molecule, which is presented as the resonance hybrid
Br
Br
Br
Br Br
“Resonance hybrid” Br
Benzene - Resonance
We can represent benzene as the hybrid of two equivalent Kekulé structures – each makes an equal contribution to the hybrid, and thus the C-C bonds are neither double nor single, but something in between
You need to understand how to “Push Arrows” If this does not make sense to you….PLEASE …get help immediately and don’t feel “dumb” O
O
Benzene - Resonance
Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds
One way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene that we saw earlier
This theory provides an explanation for the reactivity of benzene
Kekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6 with alternating single and double bonds. H H
H
H
H H
Kekulé Formulation of Benzene Note bond lengths
H
H H H H
H
H
H
H
H
H H
Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. H H H
H
H
H
H
H
H
H
H
H
Kekulé Formulation of Benzene However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. X X H
X
H
X
H
H
H
H
H
H
Structure of Benzene Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length.
Benzene has the shape of a regular hexagon.
All C—C bond distances = 140 pm
140 pm 140 pm 140 pm
140 pm
146 pm
140 pm 140 pm
134 pm
140 pm is the average between the C—C single bond distance and the double bond distance in 1,3butadiene.
